Trap Integrity : Seal Breach Risk

Hydrocarbon migration through a seal is a function of several  processes i.e. membrane leakage where petroleum buoyancy pressure > capillary entry pressure in the seal, and hydraulic leakage through fractures generated as a result of the fracture pressure being exceeded in the seal.

The actual ‘efficiency’ of the seal will be at least in part a result of pressures in the reservoirs. Where permeabilities of the sealing lithology are very low, hydraulic properties are most likely to be the deciding factor in terms of trap integrity.

Fracture pressures and gradients can be derived by analysis of Leak-Off Test data, which can then be compared with fluid pressure, derived from direct pressure measurements such as RFT data, to analyse minimum horizontal stress and seal breach risk.

Fluid pressures which are close to the fracture gradient tend to occur where either a secondary overpressure mechanism operates (such as gas expansion) or deep overpressures are conveyed to shallower levels, for example along a tilted reservoir.

A note of caution....pressures may not be equal to the fracture pressure of the surrounding shale (giving little kick tolerance) today, but may in the past have been higher, creating the possibility of mechanical seal failure and hydrocarbon escape.

Focused analysis can lead to assessment of present seal breach risk, establish a rationale for prediction of fracture gradient in undrilled locations and document the relationships between column height, pore pressure and fracture gradient.

    Consultancy- Opportunity for risk assessment

GeoPressure Technology has experience in the evaluation seal breach risk and can offer to conduct studies either on a well by well basis and/or on a more regional basis, either as a single-company consultancy or as a multi-company project with a view to:

• Reducing drilling risk with associated time and cost savings.
• Potentially increasing reserves close to existing facilities plus new exploration potential.
• Improving understanding of the regional pressure regime and its part in controlling aspects of the petroleum system, including trap integrity and reservoir quality. 
• Providing a future source of quality, reliable pressure data and accompanying information.

GeoPressure Technology has widespread, world-wide expertise in the utilisation and interpretation of pressure data and completed non-proprietary studies in the Central North Sea and Mid-Norway area investigating fracture gradients and seal breach risk in HP/HT conditions. New algorithms to calculate fracture gradient at depth incorporate pore pressure stress/coupling and aquifer and hydrocarbon seal capacity are calculated separately reflecting the two phase fluid system.

                                   Study WorkFlow

• Conversion of available pressure data into standard PressureView 4.0 database format.
• Analysis of pore fluid and fracture pressure data for all wells.
• Determination of overpressures and mapping of the distribution of formation pressures across the area at all main reservoir horizons.
• Determination of fracture strength and lithostatic stress regionally and the creation of appropriate predictive fracture gradient and lithostatic stress algorithms.
• Determination of seal integrity at all relevant horizons in all wells, plus associated seal integrity mapping where structure data provided.
• Report of key findings.  

Please contact us via our website (‘contact us’) or call Richard Swarbrick or Stephen O’Connor at the UK office (+44-191-334-2191) or Rick Lahann in US office (+1-812-606-4920).